Thick film materials are mixtures of metal, glass and/or ceramic powders dispersed in an organic medium. These materials, which are applied to nonconductive substrates to form conductive, resistive or insulating films, are used in a wide variety of electronic and light electrical components.
The properties of individual thick film compositions depend on the specific constituents which comprise the compositions. Most thick film compositions contain three major components. The conductive phase determines the electrical properties and influences the mechanical properties of the final film. The binder, usually a glass and/or crystalline oxide, holds the thick film together and bonds it to the substrate; and the organic medium (vehicle) is the dispersing medium which influences the application characteristics of the composition and particularly its rheology.
High stability and low process sensitivity are critical requirements for thick film resistor compositions for microcircuit applications. In particular, it is necessary that resistivity (R.sub.av) of the films be stable over a wide range of temperature conditions. Thus, the thermal coefficient of resistance (TCR) is a critical variable in any thick film resistor composition. Because thick film resistor compositions are comprised of a functional or conductive phase and a permanent binder phase, the properties of the conductive and binder phases and their interactions with each other and with the substrate affect both resistivity and TCR.
Functional phases based on ruthenium chemistry form the core of conventional thick film resistor compositions.
Ruthenium compounds based on the pyrochlore family have a cubic structure with each ruthenium atom surrounded by six oxygen atoms forming an octahedron. Each oxygen atom is shared by one other octahedron to form a three-dimensional network of Ru.sub.2 O.sub.6 stoichiometry. The open areas within this framework are occupied by large cations and additional anions. A wide range of substitution in this secondary lattice is possible which makes for a great deal of chemical flexibility. The pyrochlore structure with the general formula A.sub.2 B.sub.2 O.sub.6-7 is such a flexible structure. Pyrochlores which behave as metals, semiconductors or insulators can be obtained through controlled substitution on available crystallographic sites. Many current pyrochlore-based thick film resistors contain Bi.sub.2 Ru.sub.2 O.sub.7 as the functional phase.
Ruthenium dioxide is also used as the conductive phase in thick film resistor compositions. Its rutile crystal structure is similar to that of pyrochlore in that each ruthenium atom is surrounded by six equidistant oxygen atoms forming an octohedron. However, in the rutile structure, each oxygen is shared by three octahedra. This results in a complex three-dimensional network in which, in contrast to the case of pyrochlore, chemical substitution is very limited.
A problem with ruthenium-containing resistors is that, while they are superior with respect to electrical properties when compared with other materials, they nevertheless tend to cause staining of the conductive metal terminations with which they are used. In particular, it is found that when such resistors contain significant amounts of ruthenium (e.g., 5% or higher, basis solids), the associated conductive termination layers of the resistor are frequently stained with a dark, black residue which renders the termination difficult to solder effectively.